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In this post we are going to understand how LC oscillator circuits functions and we will be constructing one of the popular LC based oscillator - Colpitts oscillator.

By Girish Radhakrishnan

What are Oscillators

Electronic oscillators are used in most of our daily used electronic gadgets ranging from digital clock to high end core i7 processor. Oscillators are heart of all digital circuits but, not only digital circuit employee oscillators but also analogue circuits uses oscillatory circuits.

For instant AM, FM radio, where the high frequency oscillation is used as carrier signal to transport message signal.

There are many different kinds of oscillators such as RC, LC, crystal etc. Each one of them has their own advantages and disadvantages. So there is nothing called best or ideal oscillator, we have to analyse the circumstance of our circuit and choose the best one which suit, that’s why we find wide range of oscillators in every day used gadgets.

LC Oscillators

Let’s dive into the explanation of LC oscillator.

The LC oscillator consists of an inductor and a capacitor as shown in figure below.

The value of the capacitor and resistor determines the output oscillation. So how do they generate oscillation?

Well, we need to apply external energy between L and C i.e. voltage. When we apply the voltage, the capacitor gets charged-up. When the supply is cut-off, the stored energy from capacitor flows to inductor and inductor starts building magnetic field around it until the capacitor completely gets discharged.

When the capacitor is fully discharged, the magnetic field around inductor collapse and induces voltage and charge-up the capacitor with opposite polarity and the cycle repeats.

The charge and discharge between L and C produces oscillation and this oscillation is called resonance frequency. However the frequency generation won’t last forever due to parasitic resistance which dissipates the energy in the oscillatory circuit in the form of heat.

To maintain the oscillation and use the oscillation with reasonable output strength, we need an amplifier with zero degree phase shift and feedback.

The feedback feed small amount of output from amplifier back to LC network to compensate the loss due to parasitic resistance and maintain the oscillation. Thus we can generate steady sine wave output.

Application circuit:

Here is a colpitts oscillator circuit which can generate around 30Mhz signal.

A blocking oscillator is one of the simplest form of oscillators which is able to produce self sustaining oscillations through the use of just a few passive and a single active component.

The name "blocking" is applied due to fact that the switching of the main device in the form of a BJT is blocked (cut-of) more often than it's allowed to conduct during the course of the oscillations, and hence the name blocking oscillator.

Where a Blocking Oscillator is Typically Used

This oscillator will generate a square wave output which can be effectively applied for making SMPS circuits or any similar switching circuits, but cannot be used for operating sensitive electronic equipment.

The tone notes generated with this oscillator become perfectly suitable for alarms, morse code practice devices, wireless battery chargers etc. The circuit also becomes applicable as strobe light in cameras, which can be often seen just before clicking the flash, this feature helps in reducing the infamous red eye effect.

Due to its simple configuration, this oscillator circuit is widely used in experimental kits, and the students find it much easier and interesting to grasp it details quickly.

How a Blocking Oscillator Works

The concept of a blocking oscillator is actually very flexible, and the outcome from it can be extensively varied, simply by varying the characteristics of the involved components such as the resistors, the transformer.

The transformer here specifically becomes a crucial part and the output waveform heavily depends on the type or the make of this transformer. For example when a pulse transformer is used in a blocking oscillator circuit, the waveform attains the shape of rectangular waves consisting of rapid rise and fall periods.

The oscillating output from this design become effectively compatible with lamps, loudspeakers, and even relays.

A single resistor can be seen controlling the frequency of a blocking oscillator, and therefore if this resistor is replaced with a pot, the frequency becomes manually variable and can be tweaked as per the users requirement.

However care should be taken not to reduce the value below a specified limit which could otherwise damage the transistor and create unusually unstable output waveform characteristics. It is always recommended to position a safe minimum value fixed resistor in series with the pot to prevent this situation.

Circuit operation

The circuit works with the help of positive feedbacks across the transformer by associating two switching time periods viz, the time Tclosed when the switch or the transistor is closed, and the time Topen when the transistor is open (not conducting). The following abbreviations are used in the analysis:

t, time, one of the variables

Tclosed: instant at the end of the closed cycle, initialization of the open cycle. Also a magnitude of the time duration when the switch is closed.

Topen: instant at the every end of the open cycle, or the beginning of closed cycle. Same as T=0. Also a magnitude of the time duration whenever the switch is open.

Vb, supply voltage e.g. Vbattery

Vp, voltage within the primary winding. An ideal switching transistor will allow a supply voltage Vb across the primary, therefore in an ideal situation Vp will be = Vb.

Vs, voltage across the secondary winding

Vz, fixed load voltage resulting due to for e.g. by the opposite voltage of a Zener diode or the forward voltage of a connected (LED).

Im, magnetizing current across the primary

Ipeak,m, highest or the "peak" magnetizing current on the primary side of the trafo. Takes place just before Topen.

Np, the number of primary turns

Ns, the number of secondary turns

N, the ratio of winding also defined as Ns/Np, . For an perfectly configured transformer working with ideal conditions, we have Is = Ip/N, Vs = N×Vp.

Lp, primary self-inductance, a value calculated by the number of primary turns Np squared, and an "inductance factor" AL. Self-inductance is frequently expressed with the formula Lp = AL×Np2×10−9 henries.

R, combined switch (transistor) and the primary resistance

Up, energy accumulated within the flux of the magnetic field across the windings, as expressed by the magnetizing current Im.

Operation during Tclosed (time when the switch is closed)

The moment the switching transistor activates or triggers it applies the source voltage Vb over the transformer primary winding.

The action generates a magnetizing current Im on the transformer as Im = Vprimary×t/Lp;

where t (time) may be a changing with time and initiates at 0. The specified magnetizing current Im now "rides upon" any reverse generated secondary current Is which may happen to induce into the load on the secondary winding (for instance into the control terminal (base) of the switch (transistor) and subsequently reverted to secondary current in primary = Is/N).

This altering current at the primary in turn generates an altering magnetic flux within the transformer's windings; which enables quite a stabilized voltage Vs = N×Vb across the secondary winding.

In many of the configurations the secondary side voltage Vs may add up with the supply voltage Vb; due to the fact that the voltage on the primary side is approximately Vb, Vs = (N+1)×Vb while the switch (transistor) is in the conducting mode.

Thus, the switching procedure may have the tendency to acquire a portion of its control voltage or current directly from Vb while the remaining through Vs.

This implies that the switch-control voltage or the current would be "in phase"

However in a situation of an absence of a primary resistance and negligible resistance on the transistor switching, might result in a rise in the magnetizing current Im with a "linear ramp" which may be expressed by the formula as given first paragraph.

Conversely suppose there is a significant magnitude of primary resistance for the transistor or both (combined resistance R, e.g. primary-coil resistance along with a resistor attached with the emitter, FET channel resistance), then the Lp/R time constant could result in a rising magnetizing current curve with consistently dropping slope.

In both the scenarios the magnetizing current Im will have a commanding effect through the combined primary and the transistor current Ip.

This also implies that if a limiting resistor is not included the effect could increase infinitely.

However, as studied above during the first case (low resistance), the transistor might ultimately fail to handle the excess current, or simply put, its resistance might tend to rise to an extent where the voltage drop across the device might become equal to the supply voltage; causing complete saturation of the device (which may be evaluated from a transistor's gain hfe or "beta" specs).

In the second situation (e.g. inclusion of a significant primary and/or emitter resistance) the (dropping) slope of the current might reach a point where the induced voltage over the secondary winding is simply not sufficient to keep the transistor in the conducting position.

In the third scenario, the core used for the transformer might reach the saturation point and collapse which int turn would stop it from supporting any further magnetization, and prohibit the primary to secondary induction process.

Thus, we can conclude that during all the three situations as discussed above, the rate at which the primary current rises or the rate of rise of the flux in the core of the trafo in the third case, might show a dropping tendency towards zero.

Having said this, in the first two scenarios, we find that despite of the fact the primary current seems to continue its supply, its value touches a constant level which might be just equal to the supply value given by Vb divided by the sum of the resistances R at the primary side.

In such a "current-limited" condition the transformer's flux might tend to show a steady state. Except the changing flux, which might keep inducing voltage across the secondary side of the trafo, this implies that a steady flux is indicative towards a failure of induction process across the winding resulting in the secondary voltage dropping to zero. This causes the switch (transistor) to open.

The above comprehensive explanation clearly explains how a blocking oscillator works and how this highly versatile and flexible oscillator circuit may be used for any specified application and fine tuned to the desired level, as the user may prefer to implement.

In this post we learn how to make a simple sine wave inverter using bubba oscillator sine wave generator. The idea ws requested by Mr. Ritwik Naudiyal.

Technical Specifications

Hello Sir!! I am a 4th year B.Tech Student Electrical Eng.

We are trying to make pure wave sine wave inverter using PWM and bubba oscillator for our Final project.also along with it a battery charging and auto cut off circuit would be needed

We want the inverter to work for day to day purposes.We would be grateful to you if u can give a working circuit fr this.

thank You!

Circuit Schematic

The Circuit Design

The proposed sine wave inverter using bubba oscillator may be understood with the help of the following points:

The stage comprising two 555 ICs are configured as PWM generators where IC1 forms a square pulse generator for the PWMs while IC2 forms the monostable PWM generator with respect to the modulation input applied at its pin5.

The sine wave modulation input at pin5 of IC2 is ahieved with the help of a bubba oscillator created by using four opamps from the IC LM324.

The generated sine wave pulses are fixed at precise 50 Hz and fed to pin5 of IC2 via a BJT common collector for further processing.

The 50 Hz Formula

The 50 Hz for the bubba oscillator is set by selecting R precisely with the help of the following formula:

f = 1/2(3.14)RC

IC2 compares the sine wave modulations at its pin5 with the square pulses at its pin2 and generates an equivalent PWM waveform at its pin3.

The flip flop stage reqired for switching the power stage is configured through a single IC 4017 whose outputs are appropriately integarted with the two high gain high current power BJT stage formed by Darlington TIP122 and TIP35.

The pin14 of the 4017 is clocked at around 200 Hz via pin3 of IC1 in order to achieve a 50 HZ switching across the power transistors.

The PWM modulation of the above 50 Hz switching is implemented with the help of the two 1N4148 diodes connected across the bases of the tIP122 and are switched in accordance with the PWM from pin3 of IC1

Coil Winding Specifications

The coil is then connected to 0V.A Faraday shield which is a tin foil acting as a wrapper around the coil. This process leaves a small gap and care should be taken so that the foil does not wrap the entire circumference of the coil. An insulation tape is again used to wrap the Faraday shield.

A connection can be established to the Faraday shield with a piece of stiff wire wrapper around the shield, before adding the tape.

An ideal scenario would be to wire the circuit with twin-core or microphone cable, and connect the screen to the Faraday shield.

How to Set up the Circuit

Setting up the metal detector involves switching on the MW radio to pick up a whistle on a harmonic of 2 MHz.

However to note, not all harmonic works best, only the one which suits need to be used. With a suitable harmonic and the metal will alter the tone of a whistle.

A metal detector detects a large coin at 80 to 90 mm, which is quote good for a BFO detector. It can even identify discrimination between ferrous and non-ferrous metals with the rise or fall in tone.

Circuit Schematic

If you are wondering how a simple IC 555 can be used for making a powerful voltage doubler circuit, then this article will help you to understand the details and construct the design at home.

What's a Voltage Doubler

If you are new to voltage doubler concept and desire to learn the concept in-depth, we have a good elaborate article in this website explaining different voltage multiplier circuits for your reference.

Briefly, a voltage multiplier works by using only diodes and capacitors and the network may be capable of raising a small voltage input into a significantly high voltage output.

Voltage multiplier concept was first discovered and used practically by British and Irish physicists John Douglas Cockcroft and Ernest Thomas Sinton Walton, hence it is also called the Cockcroft–Walton (CW) generator.

A voltage doubler circuit is also a form of voltage multiplier where the diode/capacitor stage is restricted to a couple of stages only, so that the output is allowed to produce a voltage that may be twice of the supply voltage.

Since all voltage multiplier circuits mandatorily require an AC input or a pulsating input, an oscillator circuit becomes essential for accomplishing the results.

IC 555 Pinout Details

Circuit Schematic of Voltage Doubler using IC 555

Referring to the above example, we can see an IC 555 circuit configured as an astable multivibrator stage, which is actually a form of oscillator, and is designed to produce a pulsating DC (ON/OFF) at its output pin#3.

If you recall, we had discussed an LED torch circuit in this website, which quite identically uses a voltage doubler circuit, albeit the oscillator section is created using an IC 4049 gates.

Basically, you can replace the IC 555 stage with any other oscillator circuit and still get the voltage doubling effect.

However using IC 555 has a slight benefit since this IC is able to generate more current than any other IC based oscillator circuit without using any external current amplifier stage.

How the Voltage Doubler Stage Operates

As can be seen in the above diagram, the actual voltage multiplication is implemented by the D1, D2, C2, C3 stage, which are configured as a half-bridge 2-stage voltage multiplier network.

Simulating this stage in response to the IC 555's pin#3 situation can be a little difficult, and I am still struggling to get it running in my brain correctly.

As per my mind simulation, the working of the mentioned voltage doubler stage can be explained as given in the following points:

When the IC output pin#3 is in its low logic or ground level, D1 is able to charge C2, since it is able to get forward biased through C2 and pin#3's negative potential, also simultaneously C3 is charged via D1, and D2.

Now, in the next instant as soon as pin#3 becomes at high logic or at the positive supply potential, things get slightly confusing.

Here C2 is unable to discharge via D1, so we have a supply level output from D1, from C2, and from C3 also.

Many of the other online sites say that at this point the stored voltage inside C2, and the positive from D1 is supposed to combine with the output of C3 to produce a doubled voltage, however that does not make sense.

Because, when voltages combine in parallel, the net voltage does not increase. The voltages must combine in series to cause the desired boosting or the doubling effect.

The only logical explanation that can be derived is, when pin#3 becomes high, C2's negative being at the positive level and its positive end also held at the supply level, it is forced to produce a reverse charge pulse which adds up with the C3 charge, causing a instantaneous potential spike having a peak voltage twice that of the supply level.

If you have a better or technically more correctexplanation, please do feel fre to explain it through your comments.

How much Current can an IC 555 voltage doubler able to Supply?

Pin#3 of the IC is assigned to deliver a maximum of 200mA current, therefore the maximum peak current can be expected to be at this 200mA level, however the peaks will get narrower depending on the C2, C3 values. Higher value capacitors might enable fuller current transfer across the output, therefore make sure the C2, C3 values are optimally selected, around 100uF/25V will be just enough

A Practical Application Circuit of a Voltage Doubler Circuit

Although a voltage doubler circuit can be useful for many electronic circuit applications, a hobby based application could be to illuminate a high voltage LED from a low voltage source, as shown below:

In the above circuit diagram we can see how a IC 555 based voltage doubler is used for illuminating a 9V LED bulb from a 5V supply source, which would normally be impossible if the 5V was directly applied on the LED.

Relation between Frequency, PWM and the Voltage Output Level

The frequency in any voltage doubler circuit is not crucial, however faster frequency will help you to get better results than slower frequencies.

Similarly for the PWM range, the duty cycle should be roughly 50%, narrower pulses will cause lower current at the output, whereas too wide pulses will not allow the relevant capacitors to discharge optimally, again resulting in an ineffective output power.

In the discussed IC 555 astable circuit, the R1 can be anywhere between 10K and 100K, this resistor along with the C1 decides the frequency. C1 consequently can be anywhere between 50nF to 0.5uF.

R2 will fundamentally enable you to control the PWM, therefore this can be made into a variable resistor through a 100K pot.

If you have any doubts or something more interesting regarding this IC 555 based voltage doubler circuit, please do share it through your valuable comments.

We know that the center or band-pass operating frequency of a TSOP17XX IR sensor is assigned to a particular frequency which makes it difficult to use these sensors for designing unique or customized frequency based remote control circuits.

In this post we will try to figure out an idea for enabling these sensors to work with any desired unique frequency so that the circuit can be made entirely foolproof.

Basic Working Principle of TSOP17XX Sensor Modules

If we refer to the datasheet of the TSOP17XX IR sensor we find that the IC has some critical operating guidelines to ensure correct and optimal functioning of the sensor in response to an IR signal.

To enable correct functioning of the sensor, the IR signal must be oscillated at the devices's band pass center frequency value, and modulated at bursts of 10 to 70 cycles, with a certain gap after each cycle, as shown in the following image.

The image above clearly shows, that the IR beam from the Tx must be pulsed with the center frequency of the IC which is generally between 30kHz and 39kHx, and modulated with bursts of 10ms gap.

The TSOP responds to this center frequency signal and triggers ON, producing a replicated waveform at its output, wherein the 38kHz are leveled out into bursts of ordinary square wave pulses.

This complex operational waveform ensures increased immunity against many spurious frequencies that may be present in the atmosphere emanated from light bulbs, lke CFLs, fluorescent lamps etc.

Drawback of TSOP17XX Sensors

Although the sensor features a foolproof operation due to this complex signal reception pattern, the fixed center frequency for TSOP sensors restricts their use only to this specific frequency range, making it impossible to create unique customized IR remote control circuits using these chips.

Due to this drawback, a TSOP based remote control system can be usually operated using any common TV or DVD remote control handset, and using any of the buttons on the control unit.

However in electronics there's always a workaround for everything, and for these sensors too we can create a design which will allow us to use the IC with selected unique frequency of our choice so that the receiver is switched only through a particular compatible Tx pair, and not with any available common remote handset.

Designing a Unique Frequency Based TSOP Remote Control Circuit

From the above discussion we understood that a TSOP based sensors require bursts of 38kHz frequency, or the specified center frequency for operating, which indicates that the signal involves two frequencies in which the center frequency is constant but the burst frequency is variable, and not critical.

The idea is to capture this burst frequency in our favor, and use a filter which may recognize this frequency for triggering the output.

The filter circuit can be easily designed using an LM567 tone decoder circuit, and use it for decoding a particular burst frequency from the TSOP sensor output at the receiver side.

The basic concept can be witnessed in the following diagram.

Circuit Diagram

Circuit Description

Referring to the above circuit diagram for implementing TSOP17XX with customized frequencies, we see that it consists of 3 basic stages:

the TSOP17XX sensor stage

the LM567 based frequency detector stage

and the IC 4017 based flip flop or bistable circuit stage.

The TSOP17XX stage is configured in its standard mode, which picks up the modulated 38kHz frequency from the transmitter Tx unit and creates a pulsed square wave as indicated in the first diagram.

This output from the TSOP can be expected to carry the burst frequency in which we are interested in. This may be set to 1kHz, 2kHz or anything below 10kHz.

Now we want our LM567 tone decoder stage to detect this modulated frequency correctly, therefore we must make sure that the R1/C1 of the LM567 stage is calculated such that the internal oscillator locks into the same frequency matching the modulation frequency bursts from the TSOP output.

Once these parameters are set we can expect the LM567 to latch ON as soon as the selected frequency is detected from the TSOP78XX output, while any other modulation frequency is simply rejected.

In this way we are able to assign different unique frequencies ensuring that the receiver triggering is enabled only through the matching Tx handset and not with any common TV remote control unit.

Making the Customized Transmitter (Tx) Circuit

In the above discussion we learned how a TSOP17XX sensor can be operated with a customized frequency using a frequency detector stage, however this also means that the transmitter (Tx) will also need to be built uniquely for generating the customized IR signals.

The following figure shows how this may be implemented using a single IC 4049, and a few passive elements:

The 6 gates are all from the IC 4049, R3 can be 10K resistors while the presets can be 100K. The C1 caps will need to be selected with some practical experimentation. The diode can be a 1N4148, remaining resistors may be selected 2K2.

As can be seen the upper pair of gates along with R3, preset and C1 is configured as a free running oscillator, the lower section also has an identical stage.

The upper section is fed to an intermediate buffer gate whose output is finally connected with the transmitter IR photodiode.

The whole section is configured to generate the basic center frequency for the TSOP17XX compatibility which may range from 32kHz to 38kHz depending on the spec of the selected sensor.

The lower oscillator is supposed to be a low frequency modulating stage which can be seen integrated with the upper section through a diode. This low frequency switches the upper high frequency to generate the required "38kHz bursts" on the IR transmitter diode.

This low frequency actually becomes our unique frequency, or the intended customized remote control frequency which needs to be matched with the LM567 frequency so that the both frequencies "shake hands" during the IR communication between the Tx and the Rx units.

The low frequency could be selected from anywhere between 1kHz to 10kHz, and this selected range should be precisely set for the LM567 stage by appropriately adjusting its R1/C1 values.

This concludes our discussion regarding how to modify a TSOP17XX sensor circuit for accommodating customized special frequency ranges or uniquely selected frequency ranges for making the remote control system absolutely foolproof and personal.

If you have any doubts regarding the concept, the comment box is all yours!

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Swagatam is an ardent electronic researcher, inventor, schematic/PCB designer, manufacturer, and an avid publisher. He is the founder of https://www.homemade-circuits.com/where visitors get the opportunity to read many of his innovative electronic circuit ideas, and also solve crucial circuit related problems through comment discussion.